Linus Pauling: This simple consideration applies also to acids of other sorts: boric acid, H3BO3, is a very weak acid because it has only OH groups attached to the central atom.
A standard example of a molecule that exceeds the octet is phosphorous pentachloride,
PCl5. The conventional formula, structural formula for this molecule, is the one shown
here in which each chlorine atom has achieved the argon structure. It is known that
the molecule has the configuration of a trigonal bipyramid: two chlorines above, three
around the equator. The structure as indicated here causes phosphorous to form five
bonds and this would require that phosphorous make use of one orbital beyond the orbitals
in the argon shell, four orbitals in the argon shell.
This, however, is not the only structure that we may write for phosphorous pentachloride.
We may write a structure such as this one; three chlorines bonded around the equator,
the atoms of course must be in the same position for electronic resonance, resonance
of the bonds, and a chloride ion in this position, a positive charge on the phosphorous
atom, this P+, positively charged phosphorous, is now forming four covalent bonds using only the
four orbitals of the argon shell and one ionic bond. There are five structures of
this sort in which the five chlorine atoms are successively given a negatively charge,
given a negative charge, and a resonance structure involving all of these introduces
just about the right amount of partial ionic character to the phosphorous-chlorine
bonds, that would be sixteen percent of partial ionic character to the phosphorous-chlorine
bonds, corresponding pretty well to the difference in electronegativity of phosphorous
and chlorine in the electronegativity scale.
There is another aspect of valence theory that I should like to discuss now. This
is ligancy, or coordination, the coordination of several atoms or groups of atoms
around the central atom. For example, here in the sodium chloride crystal, we have
sodium ion that, where is it, sodium ion surrounded by six chloride ions in an octahedral
arrangement, chloride ions surrounded by six sodium ions. It is customary to refer
to the cation usually as the coordinating ion and to say that sodium has ligancy six
in the sodium chloride crystal. Its ionic valence is one. We can say that it forms
six, one-sixth ionic bonds with the six surrounding chloride ions. In beryllium,
in the hydrated beryllium ion, BeH2O four times, there are four bonds formed between beryllium and the surrounding water
molecules. In the hydrated magnesium ion, Be, or MgH2O six times, there are six water molecules around the magnesium ion located at the
corners of an octahedron. In BeH2O four times, they are at the corners of a tetrahedron. Now, the bonds between the
beryllium ion, or the water, or the magnesium ion, or aluminum ion in AlH2O six times, and the oxygen molecule, involves the electrons pairs, an unshared electron
pair of the water molecule. These bonds are not, however, normal covalent bonds.
They are covalent bonds with partial ionic character.
Here we have beryllium, here magnesium and aluminum in the electronegativity scale,
and oxygen over at 3.5, nitrogen at 3.0. With a metal ion and oxygen or nitrogen
in the water molecule or the ammonia molecule, the bonds have only one-third to one-half
covalent character, two-thirds to one-half ionic character so that there is not a
great amount of electric charge transferred from the oxygen atom of water or the nitrogen
atom of ammonia to the central atom. In fact, the amount of partial covalent character
of these bonds is just about enough to neutralize the charge on the central atom,
to leave it electrically neutral. We may say that there is a sort of electro-neutrality
principle operating here, that atoms strive to have zero charge rather than the charge
of plus two for beryllium and magnesium, plus three for aluminum.